The closed conduit, or surface contact system, consists of a series of plates or studs placed along the track a few feet apart and flush with the road, and insulated from each other. Under ordinary circumstances these are disconnected with the conductor, which is laid entirely below the surface, but when a car passes over them they become, by means of switches, automatically connected with it, so that the current can be conveyed through them to the car motors. In other words, the studs are “alive” while the car is over them, and “dead” as soon as it has passed. This is a very practicable method, and in certain cases is preferable to the open conduit. Defects, however, there are, but the Dolter apparatus claims to have overcome them, and it is greatly in its favour that the system has been successfully worked in Paris for more than two years. It has the merit of readily lending itself to a combination with the overhead trolley system. Of all systems, by far the best known to the public is that of “overhead,” recognised immediately by the tall iron poles inseparable from its adoption. Ninety-five per cent. of the world’s electric tramways are worked on the overhead principle. The distribution of electric energy is by means of a wire, called the trolley wire, upheld by insulated brackets on poles twenty feet above the ground, along the entire track, which is divided into sections, each section taking its current from the main conductor-wire, which is laid underground, through the iron poles. Should any one section of the trolley wires meet with mishap, only the cars working on that section are stopped; those on the remaining divisions, having an independent source of current, continue to run [Illustration: FIG. 16. CROSS LANE JUNCTION, SALFORD. THE LARGEST AND MOST COMPLICATED OVERHEAD TROLLEY CROSSING IN THE KINGDOM _By permission of_ _Geo. Hill & Co., Manchester_] without interruption. At the upper end is a small deeply grooved wheel which, by means of springs at the base of the trolley pole, is pressed against the under side of the trolley wire overhead, and in that position remains as the car proceeds. From the wire the electric current passes through the grooved wheel and down the trolley pole to the motors, of which there is one at each end of the car.